1. Field of the Invention
The present invention relates to a compact zoom lens suitable for a lens shutter type compact camera and, more particularly, to a wide-angle zoom lens including a wide field angle of about 70.degree. or more.
The present invention also relates to a focusing method of a zoom lens suitable for a photographing lens integrated type auto-focus camera and a video camera.
2. Related Background Art
In recent years, even compact cameras generally have zoom lenses, and many zoom lenses constituted by two, i.e., positive and negative, groups have been proposed in, e.g., Japanese Laid-Open Patent Application No. 2-73322 (a counterpart application of U.S. Pat. No. 4,929,069).
A zoom lens proposed in Japanese Laid-Open Patent Application No. 2-73322 has a field angle of about 60.degree. at the wide-angle end. However, even a zoom lens for a compact camera is required to have a wider field angle. In order to meet such a requirement, zoom lenses including a wide-angle region of 70.degree. or more have been proposed in Japanese Laid-Open Patent Application No. 3-240013 (a counterpart application of U.S. Pat. No. 5,126,884), Japanese Laid-Open Patent Application Nos. 5-19166 and 5-232383, and the like.
However, in the respective embodiments of Japanese Laid-Open Patent Application No. 3-240013, since a zoom lens is constituted by many lenses, i.e., 11 lenses, it is difficult to achieve a low-cost, compact zoom lens. In the respective embodiments of Japanese Laid-Open Patent Application Nos. 5-19166 and 5-232383, the total lens length at the wide-angle end (the distance from a lens surface closest to the object side to an image plane) is as large as 1.9 times or more the focal length at the wide-angle end, and the zoom lens cannot be made sufficiently compact.
In the respective embodiments of Japanese Laid-Open Patent Application Nos. 5-19166 and 5-232383, the number of groups (a single lens and a cemented lens are respectively counted as one groups) constituting a first lens group is four or more, and assembling and adjustment are not easy.
In the arrangement described in each of the first to seventh embodiments of Japanese Laid-Open Patent Application No. 2-73322 mentioned above, a positive first lens group, which is arranged at the object side of a stop, is moved as a whole in the object direction to achieve a focusing operation to a near distance. In the arrangement described in the eighth embodiment of this reference, the first lens group is constituted by a positive front group and a positive rear group to sandwich a stop therebetween, and the positive front group on the object side of the stop is moved to achieve a focusing operation to a near distance. However, when a zoom lens in which a lens group closest to the object side is moved upon focusing, like a zoom lens proposed in, e.g., Japanese Laid-Open Patent Application No. 2-73322, is mounted on a camera, sufficient mechanical strength must be provided to a focusing mechanism, so that focusing precision is not influenced by an external force, and the focusing mechanism does not fail. As a result, cost increases. As a method of preventing an external force from acting on the focusing mechanism, a filter glass may be fixed in front of a focusing lens group.
However, with this method, an increase in cost due to addition of a filter glass member is inevitable.
On the other hand, in the case of a so-called inner focus, rear focus type lens, which performs a focusing operation using a lens group behind a lens group closest to the object side, since the focusing moving amount changes depending on the zoom position, complicated calculation formulas and many numerical value tables must be stored to calculate the focusing moving amount, and cost increases due to an increase in ROM capacity.